This invention relates to a method of measuring the relative density value of a gas.
Relative density is a dimensionless number.
The gas may be a fuel gas, for example natural gas. The natural gas may be methane and may further comprise nitrogen and/or carbon dioxide. In addition to methane the natural gas may comprise at least one other hydrocarbon gas, for example ethane, propane, butane, pentane or hexane.
According to a first aspect of the invention a method of measuring the relative density of a gas comprises making a measure of the speed of sound in the gas, making a measure of a first thermal conductivity of the gas at a first temperature, making a measure of a second thermal conductivity of the gas at a second temperature which differs from the first temperature, and using the speed of sound and the first and second thermal conductivities in an operation producing the relative density of the gas corresponding to said speed of sound and said first and second thermal conductivities.
According to a further aspect of the invention an apparatus for measuring the relative density of a gas comprises means for making a measure of the speed of sound in the gas, means for making a measure of a first thermal conductivity of the gas at a first temperature, means for making a measure of a second thermal conductivity of the gas at a second temperature which differs from the first temperature, and means for using the speed of sound and the first and second thermal conductivities in an operation producing the relative density of the gas corresponding to said speed of sound and said first and second thermal conductivities